<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>flex演示</title>
    <style>
        .demo{
            display: flex;
            flex-direction: column;
        }
        .demo div{
            flex: 1;
        }
        .body{
            display: flex;
        }
        .header, .footer{
            background: grey;
            line-height: 80px;
            text-align: center;
        }
        .left, .right{
            background: pink;
            line-height: 400px;
            text-align: center;
        }
        .header,.footer,.left,.right{
            flex: 0 0 20%!important;
        }
        .left {
            flex-direction: column;
        }
    </style>
</head>
<body>
<select>
    <option>aaa</option>
    <option>aaa</option>
    <option>aaa</option>
</select>
    <div class="demo">
        <div class="header">头部</div>
        <div class="body">
            <div class="left">
                <div>aaa</div>
                <div>aaa</div>
                <div>aaa</div>
                <div>aaa</div>
                <div>aaa</div>
            </div>
            <div class="center">center</div>
            <div class="right">right</div>
        </div>
        <div class="footer">底部</div>
    </div>

</body>
</html>